Polytetrafluoroethylene lubricant for carbon composition resistors



Nov. 14, 1957 e. F. CH WICK 3 353,]35v POLYTETRAFLUOROETHY E LUBRICANTFOR I CARBON COMPOSITION ISTORS Filed June 14 65 NVENTo GEORGE F. CHADwBY MW ORNEY United States Patent 3,353,135 POLYTETRAFLUOROETHYLENELUBRICANT FOR CARBON COMPOSITION RESISTORS George F. Chadwick, NorthTonawanda, N.Y., assignor to Air Reduction Company, Incorporated, NewYork,

N.Y., a corporation of New York Filed June 14, 1965, Ser. No. 463,846 7Claims. (Cl. 338226) ABSTRACT OF THE DISCLOSURE A carbon compositionresistor including by weight 0.2% to 1% polytetrafluoroethyleneinterdispersed in the core and shell thereof. The additive citedincreases heat stability and load life properties of the resistor andprovides lubrication during resistor molding.

The present invention relates to electrical resistors and moreparticularly the invention relates to electrical resistor compositionshaving high heat stability and other advantageous properties.

Carbon composition resistors are widely used in a great number ofindustries, and particularly in the radio and television industries.Such resistors must be able to withstand high temperatures for longperiods of time without a great variation in resistance values. To thisend, efforts are constantly being :made by the resistor manufacturingindustry to improve the operating characteristics of such resistors.

Prior art carbon composition resistor conductive cores generally consistof a nonconductive filler material, such as silica, a conductivecarbonaceous material, such as carbon black, lampblack, graphite, etc.,and an organic binder which may include coal pitch phenolic resins,silicone resin, acetone, etc. The proportions of the ingredients arevaried to obtain the desired resistance values and electricalproperties. An insulating shell usually en capsulates the conductivecore, the shell generally comprising a binder of organic resins such asphenolic or silicone resins, and a filler material such as silica sand.

I have found that by the use of small percentages ofpolytetrafluoroethylene dispersed in such resistor cores and shells, Iachieve a carbon composition resistor having increased heat stabilityand greatly extended load life.

It is an object of the present invention to provide an improved resistorcomposition. It is another object of the invention to provide animproved carbon composition resistor.

Another object of the invention is to provide a carbon compositionresistor having superior heat stability and longer load life.

Other objects, features, and advantages of this invention will bereadily apparent from consideration of the following specificationrelating to the annexed drawing in which:

FIG. 1 is a longitudinal cross-sectional view of the carbon compositionresistor of this invention;

FIG. 2 is a plan view of the shell portion of the resistor of FIG. 1prior to insertion of the core portion of the resistor into the shell;and

FIG. 3 is a plan view of the core portion of FIG. 1.

FIGURES 1-3 are representative showings of carbon composition resistorswhich may employ my novel resistor compositions. In the manufacture of acarbon composition resistor it is conventional to preform a carboncomposition core portion such as shown at 1 in FIG. 3. The core isprovided with terminal sockets as shown at 7. The core is then insertedin a preformed resistor shell such as shown at 2 in FIG. 2. Suitableelectrical terminals are inserted into sockets 7 and the assembled shelland terminals are then pressed in a conventional manner,

either with or without the presence of heat to form a resistor as shownat 3 in FIG. 1. The completely formed resistor thus has terminals asshown at 4, a core portion as shown at 5, and a shell portion as shownat 6 in FIG. 2. The core is the conductive portion of the resistor andthe shell serves as an insulating and protective nonconductive jacket.In some instances the shell may not be necessary or desired, in whichcase the resistor consists merely of the core portion and terminals ofFIG. 1. A satisfactory method of producing the resistor of FIG. 1 isfully described in United States Patent No. 2,471,592. Other methods ofmanufacturing a carbon composition may also advantageously employ mynovel carbon resistor core and/ or shell compositions.

A serious problem encountered by the use of prior art carbon compositionresistors has been the inability of the resistor to maintain itsoriginal resistance value under high operating temperatures for longperiods of time. I have discovered that the addition of 0.2% to 1% ofmicronized polytetrafluoroethylene to the resistor core mix and 0.5% to2% micronized polytetrafluoroethylene added to the resistor shell mixgreatly increases the heat stability and load life of carbon compositionresistors.

It should be noted that all references to percentages in thisapplication refer to percentages by weight of the total composition ofthe core or shell.

Amounts of polytetrafluoroethylene in substantial excess of 1% in theresistor core mix are detrimental to the resistor characteristics inthat the RTC (resistance/ temperature characteristics) requirements ofthe resistor are exceeded. At the same time, an additive of greater than2% polytetrafluoroethylene in the resistor shell mix does not appear toimprove the heat stability and load life of the resistor.

It should be noted that although the percentages ofpolytetrafluoroethylene set forth hereinabove are the percentages addedto the resistor core and shell mix respectively, the percentage ofpolytetrafluoroethylene is substantially the same in the finishedresistor core and shell. The resistor loses only approximately 2% byweight in volatiles during the curing and other finishing processes andsince the polytetrafluoroethylene is such a small percentage of theoverall resistor formulation, the proportion of polytetrafluoroethyleneto the total resistor weight does not substantially vary from the timeit is added to the core and shell mix until such time as the resistor isfinished. Therefore, the percentages set forth in this application areintended to refer to the proportion of polytetrafluoroethylene presentin either the finished core or shell.

Table 1 compares the percentage change in the resistance of carboncomposition resistors having centage of micronizedpolytetrafluoroethylene added to the resistor core and resistor shellmixes with the percentage change in the resistance of resistors havingthe same composition but without polytetrafluoroethylene. In the test, anumber of identically manufactured resistors, except for the presence orabsence of polytetrafluoroethylene as indicated, were tested for thespecified time and temperature and theresistance change shown in thetable is the average resistance change for the total number of resistorstested.

TABLE 1 Average resist. Average resist. Operating Operating change withchange without Temp. C.) Time (i112) PIFE P'IFE (percent) (percent) 1500. 61 +0.02 150 150 0. S7 +0.16 150 500 0. -4. 73 750 1. 33 I4. 02 I 1501,000 l. 57 20. 94

*PTFEzpolytetrafinoroethylene.

a small per-.

2' 6 The composition of the core mix for the resistor withpolytetrafiuoroethylene tested in Table 1 consists of the followingformulation:

The core mix of the resistors without polytetrafluoroethylene wasmanufactured from the same composition as set forth above except thatthe polytetrafluoroethylene wasomitted.

The, shell mix'formulation for the resistors withpolytetrafluoroethylene tested in Table 1 is as follows:

Base shell mix: Grams 330 lb. silica sand, 120 lb. phenolic resin, .5lb.

silicone resins 1000 Polytetrafluoroethylene 10 Total 1010 The shellmix' used for the resistor without polytetrae fluoroethylene was simplythe base shell mix:set'forth above.

It is apparent from Table 1 that resistor compositions not containingpolytetrafluoroethylene dispersed therein are subjected to vastlygreater resistance changes when tested at high; temperatures over a longperiod of time. It .is believed, but not certain, .that.the reason forthe. improved heat stability of carbon composition resistors containingpolytetrafluoroethylene lies in the slower'oxygen penetration into the.resistor core.

It should be noted that. the. particular formulations recited areconventional except for the polytetrafluoroethylene additive and areintended only to be exemplary ofcompositionsfor carbon compositionresistors which may utilize polytetrafluoroethylene as an additive toincrease load lifeand heat stability. Polytetrafluoroethylene mayobviously be used to improve the operating properties of other carboncomposition resistors when added to the resistor. formulations in thequantities. specified in this application.

In addition to the advantages given above, it is found that the additionof micronized polytetrafluoroethylene to the core or shell mixsubstantially lowers the ejection force required to remove the shell orcore from its particular die or mold cavity. For example, Table 2 listssixcarbon composition resistors'having different known internallubricants, which lubricants are identified'in the table, and a seventhresistor having polytetrafluoroethylene in the mix. It can be seen fromTable 2' that the ejection force requiredto eject the compositioncontaining thepolytetrafluoroethylene is markedly reduced-when compared.to the ejection'pressure necessary to remove the resistor compositionscontaining other lubricants from the mold.

Table'2 Lubricant: Ejection force Carbowax 1100 Polyethylene 350 Calciumstearate 900' Lithium stearate 750 Aluminum stearate 1100 Zinc stearate1200 Polytetrafiuoroethylene 70 Obviously many modifications andvariations of the is therefore to be understood that within the scope ofthe appended claims, the invention may be practiced otherwise than asspecifically disclosed.

I claim:

1. .A carbon composition resistor comprising a conductive core memberhaving 0.2% to 1% by weight polytetrafiuoroethylene dispersed therein,electrical terminal means attached to said core member, and aninsulating jacket surrounding said core member.

2. A carbon composition resistor comprising a conductive member having0.2% to 1% by weight polytetrafluoroethylene dispersed therein, andelectrical terminal means connected to said member.

3. A carbon composition resistor comprising aconductive core memberhaving 0.2% to 1% by weight polytetrafluoroethylene dispersed therein,electrical terminal means connected to said core member, and aninsulating jacket having 0.5 to 2%-by weight 'polytetrafluoroethylenedispersed therein .surroundingsaid core member.

4. A carbon composition resistor comprising a conductive core member,electrical terminal means connected to said core member, and aninsulating,jacketsurrounding said core member, the composition of saidconductive core member comprising a nonconductive filler material, acarbonaceous conducting material, an organic binder, and interdispersedquantities of polytetrafiuor-oethylene, the

total amount of polytetrafiuoroethylene present in said compositionbeing between 0.2%. and 1%" by weight.

5. A'carbon composition'resistor comprising a conductive member andelectrical terminal means connected to interdispersed quantities ofpolytetrafiuoroethylene, the. total amountof polytetrafiuoroethylenepresent insaid' composition being .between'0.2% and 1% by'weight, andsaid insulating'jacket having 0.5% to 2% by weightpolytetrafluoroethylene dispersed therein.

7. .A carbon composition resistor comprising a conduc-q tive coremember, electrical terminal meansconnectedto said core member, and aninsulating jacket's'urounding said core member, the-composition of saidconductive core member comprising a nonconductive fillermateriaLacarbonaceous conducting material, an organic binder,andinterdispersed quantities .of, polytetraflu'oroethyle'ne, the totalamount of polytetrafluoroethylene presentinsaid composition beingbetween 0.2% and.l% byweight, and

thecomposition of said. insulating jacket comprisingsilica:

sand, an organic binder, and interdispersed:quantities ofpolytetrafluoroethylene, the total amount of polytetrae fluoroethy'lenepresent in the composition of saidv insulating jacket beingbetween 0.2%and 1% by weight.

References Cited UNITED STATES PATENTS 2,744,988v 5/1956 Tierman 338-322X 2,883,502 4/1959 Rudner 338330 3,037,266 6/1962 Pfister 338.-226 X.

RICHARD M. WOOD, Primary Examiner.

I. G. SMITH, Assistant. Examiner.

1. A CARBON COMPOSITION RESISTOR COMPRISING A CONDUCTIVE CORE MEMBERHAVING 0.2% TO 1% BY WEIGHT POLYTETRAFLUOROETHYLENE DISPERSED THEREIN,ELECTRICAL TERMINAL MEANS ATTACHED TO SAID CORE MEMBER, AND ANINSULATING JACKET SURROUNDING SAID CORE MEMBER.